Angle modulation detector



Feb. 14, 1950 s. w. SEELEY 2,497,840

ANGLE MODULATION DETECTOR Filed June 14, 1945 2 Sheets-Sheet 1ZIFAMH/HEQ ZS j I Abb t/F752 Ply 2/6.

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IN V EN TOR \fru/wrldd SEA-2 0 ATTORNEY Patented Feb. 14, 1950 UNITEDANGLE MODULATION DETECTOR Stuart W. Seeley, Roslyn Heights, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication June 14, 1945. Serial No. 599,343

34 Claims.

My present invention relates generally to detectors of angle modulatedcarrier waves insensitive to amplitude variations, and more particularlyto a. novel method of, and means for, deriving the modulation from afrequency modulated (FM), or phase modulated (PM), carrier wave withoutallowing co-existent amplitude modulation (AM) variations to result insubstantial detector output potentials.

In the past there have been provided various methods of detecting anglemodulated carrier waves without response to undesired amplitudevariations. By angle modulation" is meant either FM, PM or hybrid formsof modulation possessing characteristics common to both of them. In thegeneration, transmission and reception of angle modulated waves suchundesired AM effects may come from the transmitter directly, may be dueto interfering impulses, or may be caused by lack of uniform gain overthe signal selector pass band. In prior detection systems which wereinherently immune to such undesired AM effects the means employed waseither relatively uneconomical as compared to the cost of a specialamplitude limiter stage prior to the detector, or the degree of immunityto undesired AM effects was insuificient.

It is, therefore, an important object of my present invention to providea simplified detector of angle modulated waves which has a high degreeof immunity to undesired AM effects of received waves, the detectorincluding a minimum of special components over and above those requiredfor known forms of detectors, and which improves the operation of FMreceivers.

It is well known that most balanced FM discriminators are balancedagainst amplitude variations at the center frequency of theircharacteristic. However, if the mean frequency of the received FMsignals shifts relative to center frequency, spurious AM effects maycause either distortion. interference or both. It is another importantobject of my present invention to provide an FM, or other anglemodulation, detector which is balanced against such AM interference ordistortion regardless of whether the received carrier (or even the AMcomponent of the waves) is centered on the detector characteristic.Another important advantage of a balanced FM detector circuitconstructed in accordance with my 2 invention is that "interstation"noise eflects may be considerably reduced.

A further object of my invention is to provide a demodulator forfrequency or other anglemodulated waves including a pair of rectifiers,means for combining the rectified current potentials of the rectifiersin series-aiding polarity, and means for connecting the rectifiers inparallel for derivation of modulation frequency waves and forsubstantially maintaining the sum of the rectified potentials againstmodulation frequency variations for rejection of undesired amplitudevariations of the modulated waves impressed on the rectifiers.

In accordance with a still further object of my invention, I provide afrequency modulation detector comprising a pair of rectifiers eachhaving at least an anode and a cathode, a circuit con necting the anodeof one rectifier to the cathode of the other, a second circuitconnecting the cathode of the first-mentioned rectifier to the anode ofthe second-mentioned rectifier, said circuits in series with saidrectifiers constituting a closed loop for flow of uni-directionalcurrents, in combination with means for applying frequency modulatedcarrier waves to the respective rectifiers, and means, which mayconveniently be resistance and a large capacity connected in parallelwith each other, in one of the circuits for establishing a directcurrent potential substantially fixed against modulation frequencyvariations, and connections for deriving modulation frequency voltagevariations from one of the circuits in relation to the other.

In accordance with a still further object, my invention contemplates inthe detection of anglemodulated waves the method which includesimpressing respectively upon a pair of rectifiers a pair of signalpotentials whose relative magnitudes are a function of the anglemodulation of the received waves, adding the respective rectifled directcurrent potentials in series-aiding relation, maintaining the total ofthe added potentials against modulation frequency variations, variablydamping the input to the rectifiers as a function of the instantaneousamplitude of the signal impressed on the rectifiers, and derivingmodulation frequency variations from the rectifler circuits.

Application of my invention to a known form of balanced FM detector ofthe general type disclosed and claimed in my U. 8. Patent No. 2.121.103,granted June 21, 1938, involves reversal of one of the diode rectiilers.Such reversal results in the rectified direct current voltages acrossthe diode load resistors. now in series. becoming additive, instead ofproviding zero direct current voltage at the output point in response tothe FM carrier frequency being accurately centered. The additive voltageis either positive or negative depending on which end of theseries-connected load resistors is grounded. The ungrounded end of theseriesrelated load resistors is then established at a relatively fixedpotential for modulation signal frequencies whereby substantially nomodulation voltage can exist between the opposite ends of the loadresistors. In effect. then, the load resistors are in parallel to groundfor modulation voltage, but in series for direct current voltage. Thetap between the two load resistors, or a variable tap on either one, maythen serve as the modulation voltage output point.

In a further important embodiment of my invention, the direct currentvoltage across the series-connected load resistors is fixed in apredetermined manner, so that the direct current voltage cannot varysubstantially at a modulation rate. There is thereby established athreshold value below which no detection of any kind can occur.

It is a further object of one embodiment of my present invention toprovide an FM detector circuit having the aforesaid functions, and, inaddition, a de-emphasis effect due to a suitable time constant networkprovided by the discriminator condensers and the load resistors.

More specific objects of my invention will appear in the followingdetailed description of various embodiments of the basic features, itbeing pointed out that my balanced FM detector system, substantiallyimmune to AM effects, has important applicability to low-cost FMreceiver construction.

The novel features which I believe to be characteristic of my inventi-nare set forth with particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description, takenin connection with the drawing, in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings:

Fig. 1 is a schematic circuit diagram of one embodiment of theinvention;

Figs. 2a and 2b respectively show different vector relations between theprimary and secondary voltages of the discriminator;

Fig. 3 shows a modification of Fig. 1, wherein fixed bias is employed onthe diodes;

Fig. 4 illustrates a circuit arrangement for securing delayed automaticvolume control (AVG) action from the present detector circuit;

Fig. 5 shows a further modification of the detector circuit; and

Fig. 6 portrays illustrative and explanatory FM signal input amplitudevs. detected output voltage characteristics of the present invention.

Referring now to the accompanying drawings. wherein like referencecharacters in the several figures designate similar circuit elements,there is shown in Fig. 1 the detector network of an FM receiver of thesuperheterodyne type. While my invention is readily incorporated in anyform of receiver of I'M waves. I prefer to explain the functioning of myinvention in connection with a superheterodyne receiver system sincesuch a system is widely used at present. As previously explained thepresent invention is not restricted to reception of PM waves, sincephase modulated carrier waves could be received as well. An FM wave isproduced at the FM transmitter by deviating the carrier wave relative toits mean frequency to an extent proportional to the amplitude of themodulating voltage and independent of the modulating frequency. A PMwave difiers in having a frequency deviation which increases withmodulatlng frequency. The generic expression "angle modulation" is,also, intended to include a modulated wave of preferably constantamplitude wherein the modulation is neither pure FM nor pure PM, butcontains components resembling one or both of them. and is, therefore, ahybrid modulation.

In the present application it is assumed, by way of specific example.that the receiver is designed to operate in the PM broadcast band of42-50 megacycles (mo). and that each transmitter radiates an FM wavehaving a maximum frequency deviation up to :75 kilocycles (kc.) withrespect to the normal transmitter frequency. These are the assignedfrequency values of the present day FM broadcast band: and are used heremerely by way of illustration. The receiver may include any desired formof signal collector. as., for example. a dipole. The collected FM signalwaves may be applied to a suitable converter for reduction of the meanfrequency value without change of the deviation. The converter may be ofany desired construction, and is preferably preceded by one or morestages of selective high frequency amplification. Suitable signalselector 0 circuits, usually employing a variable condenser oradjustable inductor, are employed for adjustment to receive a desired FMstation. The signal selector circuits will. of course, preferably beadlusted accurately to resonate the various adjustable selector circuitsto the center or mean frequency of the desired FM station.

In a superheterodyne receiver the converter is fed with oscillationsfrom an oscillator whose tank circuit includes an adjustable reactancedevice, usually a variable condenser or adjustable inductor. The latteris customarily adjusted concurrently with the aforesaid selector devicesso that the tank circuit will be tuned to a local oscillation frequencydiflering from the desired carrier frequency by the operatingintermediate frequency (1. Ill. The selective circuits of. andpreceding, the converter may on the other hand be of the fixedly tunedtype. if desired. The intermediate frequency is usually chosen from arange of 2 to 15 me., by way of example. 5 me. Any suitable actuatingmechanism may be used for operating the station selecting devices. Theconverter may use the well-known pentagrid tube. or it may use separateoscillator and mixer tubes. These various circuits and circuitcomponents are very well known to those skilled in the art of radiocommunication. and need only be briefly referred to.

The I. I. amplifier network may embody one or more amplifier tubesselectively tuned to the operating I. 1". value of it me. Of course. allsignal transmission circuits between the signal collector and thedemodulator or detector will be so constructed as to pass efllciently aband at least kc. wide. It is, also, usual to design the signal onaco oncircuits to have a pass band of apin width to provide for reaasoscillator drift and The output transformer I feeding the secondarycircuits and 4 each tuned to the operating I. F. value. The pass band oftransformer i will preferably, as indicated above. be chosen to be ofthe order of 200 kc. wide. while the mean or center frequency of theband is 5 mc.. in the example explained.

One of in the past for employing an amplitude limiter prior to thediscriminator section (or FM translating network) of the demodulator toreduce undesired AM effects on the carrier wave. was to avoid thenecessity for critical tuning to the exact center or carrier frequencyof a desired FM wave.

In my present system no limiter stage need be employed prior to thedetector circuit, since the detector itself is substantially immune toamplitude variations of the received FM signals. Hence, the I. F.amplifier tube 2 possesses normal and full gain, which is the reverse ofthe usual operating condition for an amplitude limiter. The amplifiertube 2 may be of any suitable construction; its cathode 5 is connectedto ground through a suitably bypassed cathode bias resistor 1. Theungrounded side of input circuit I is connected to control grid 8, andplate 9 of the tube is connected to the high potential side of resonantcircuit 5. The plate a is connected to the +8 terminal of a suitablesource of direct current (not shown) through prispecial amplitude marycoil III, while condenser ii establishes the lower end of coil Ill atground potential for I. F. currents.

The discriminator comprises coupled primary and secondary circuitsdenoted by numerals 5 and i2 respectively. While any known and suitablediscriminator may be utilized to provide the energizing signal voltagesfor rectifiers l3 and it. I prefer to explain the action on the basis ofthe discriminator circuit of Fig. 1. The discriminator network isgenerally of the type claimed and shown in my aforesaid patent. and isspecifically Fig. l of application Serial No. 529,014, W. R. Koch now U.S. Patent 2.410.983. In general, it is desired to employ a networkconstructed and arranged to derive from angle modulated waves a pair ofvoltages whose relative amplitudes vary in accordance with the angulardeviations of the waves with respect to a reference condition (whetherphase or frequency).

Considering the specific illustrative embodiment of Fig. l, coil Ill isshunted by condenser i5 to provide a parallel resonant circuit tuned tothe operating I. F. The secondary coil 16 is coupled to primary coil to,as indicated by numeral i1, and coil i6 is shunted by condensers l8 andis arranged in series. The resonant secondary circuit l2, including coilis and condensers l8, I! is tuned to substantially the resonantfrequency of the primary circuit 5. Each of coils- Ill and i5 ispreferably of the inductance trimmer type. Specifically, iron cores,plugs or slugs (symbolically represented at the lower ends of the coilsill, ii) are used for adjusting values of the coils Ill and it. The highalternating potential side of coil ill is connected by a lead to thejunction of condensers Iii and is thereby establishing the junctionpoint at the same aiternatlng potential as the high potential side ofprimary circuit 5.

all

the inductance i. e.. from one Concurrently. each of condensers it andit will function as a direct current blocking condenser to preventapplication of +3 voltage to the rectiflers II and II. Moreover.condensers it and II offer a low impedance to the I. F. voltage at theprimary circuit so that substantially the full voltage is applied to therectiand il. Condensers II and is are approximately equal in value.Their combined series impedance in shunt with the wiring, tube and otherstray, capacitative impedances resonates the coil II to the operatingfrequency. If desired, no magnetic coupling need be used between coilsII and II. For example, if the circuits are coupled by augmenting thediode capacity on one side. or the other, to produce a capacityunbalonce, the primary coll ill and secondary coil is will becapacitively coupled by an amount depending upon the magnitude of theunbalance of the two capacities.

Rectifiers is and it are shown by way of specific example as electrondischarge devices of the diode type. It is to be clearly understood thatthe diodes may have their electrodes embodied in a common tube envelope,min the 636 type tube. Applicant further recognizes that in the practiceof his invention other forms of two-electrode rectiilers may be used inplace of diode vacuum tubes, and his claims assume that other suitableforms of two-electrode rectiflers can correctly be said to have an anodeand a cathode. The anode 20 of diode I3 is connected to the upperterminal, as diagrammatically shown, of condenser is and to the upperend of coil 15, whereas the cathode ii of diode It is connected to thelower terminal of condenser l9 and to the lower end of coil IS. Thecathode 22 of diode l5 and the anode 23 of diode is are directlyconnected by condenser 24. The anode 23 and the lower potential terminalof condenser 25 are established at ground potential for direct current.In accordance with my present invention. the magnitude of the condenser24 is chosen so that the cathode 22 of diode I3 is also at groundpotential with respect to modulation frequencies l. e., audio frequency,as well as for I. F.

The load resistorfor diode i3 is designated by numeral 25, and it willbe observed that the load resistor 25 is connected directly in shuntwith the space current path of diode l3. Similarly, load is alsoconnected directly between anode and cathode of its diode I4, and it is,therefore, directly in shunt with the space current path of that diode.There will be developed across each of resistors 25 and 25 respectivevoltages produced by rectification of signal currents.

Hence, it can be seen that the diodes l3 and M are arranged in reverserelation relative to the connection in a conventional FM detectorcircuit of the type employing balanced detector circuit diodes. Thismeans that the direct current voltages across load resistors 25 and 25are in additive polarity relation. Instead of providing zero directcurrent voltage when the applied carrier frequency is at balance, eithera positive or negative voltage results depending upon which end of theseries connection of the load resistors is grounded. are bypassed, as bycondenser so that little or no modulation frequency voltage can existbetween the cathode end of resistor 25 and ground. The resistors 25 and25 are effectively in parallel to ground for audio frequency voltages,which are accordingly taken off from across the rectifiers, side of therectiflers in relation relation to ground voltages.

to the other but are in series with direct current rectiflers form aclosed loop ior flow oi uni-directional currents. Resistors II and 26are connected between such circuits and either of the resistors mayserve conveniently for taking 011' a desired portion of the audiofrequency voltage.

Accordingly, the be applied to anode 20 through therefore, that quency,and the rectified voltages across respective resistors 25 and 2' will beof equal magnitude. In Fig. 2a, 1 have portrayed the Vector relationswhich exist at the instantaneous carrier frequency between the primaryvoltage E, and each The hair or the total leoon ant voltages a When Irecite in or any or them "means for applying each of to a respectiverectifier." the meaning is that one to one of the rectiflers and theother 01' the voltages is applied to the othe claims I have recited "asource or angle modulated carrier waves or "a source of frequencymodulation signal" or "a source of frequency modula like, I refer toparts of the receiver preceding the rectlfiers.

total voltage across resistors 25 and 2| would still equal +10 volts.11' the applied resistors shunt the diodes as in Fig,

or in accordance with a these frequency signals varied cyclically,desired FM program, between values, the junction of resistors II and I!(point 3i) would vary between +3 volts and +7 volts in accordance withthe frequency variations of the signal. The tap it can be adjusted totake oi! any desired magnitude oi modulation frequency voltage acrossresistor 26.

Consider, now, the action of resistor 21 which to resistors 15 and II.It can be demonstrated that ii resistors 25 and 25 are normally veryhigh (/2 to l megohm) or no 5, the two diodes in the circuit produce adamping action on the primary circuit which is the equivalent of thatwhich would result if a resistor of approximately one-eighth the valueoi that used for resistor 21 were placed directly in shunt with theprimary. However, the action on the secondary circuit is only equivalentto the placing 01 a resistor of about one-half the value or resistor 21across the secondary. Therefore, the eflect on the primary circuitincreasing or decreasing diode currents can be substantially greaterthan the action on the secondary circuit. For this reason the presentexplanation disregards any diodes may have on the secondary circuit.However, these eflects can also be altered by the relative L/C ratios ofthe primary and secondary, and another type of operation, still withinthe scope of this invention, might utilize damping action equally onboth circuits or principally on the secondary.

It is well known that any balanced discriminator is immune to amplitudevariations when such variations occur at the center irequency, and thiscircuit is no exception to that rule. However, if the amplitude of theapplied signal energy is suddenly changed while the carrier is at oneside of center frequency shifted the steady state unmodulated carrier toone side) the balance condition does not hold, and amplitude variationsin a normally balanced discriminator are reproduced as audio voltages.

In this present circuit a sudden increase in applied signal energyresults in a sudden increase in the diode currents, but the applieddiode voltages can increase only very slightly, since the bias on thetwo diodes l8 and i4 is rigidly maintained against audio fluctuations bythe bypass condenser 2|. Thus, a very minor increase in applied signalvoltage causes a very large increase in diode current, which means aning action on the primary circuit 5. However, even though the diodecurrents do increase markedly, they will increase in direct proportionto the lengths 0! E and E". Since the sum of the output voltages is heldconstant against audio fluctuations by the condenser 24, it developsthat a sudden increase in applied signal produces practically no changein the audio potential at point 3 l. The condenser 2i in conjunctionwith resistor 21 thus acts as a voltage or potential source constantagainst amplitude variations at modulation frequencies by maintainingconstant against such variations the potential produced by rectificationor the angle modulated waves.

Conversely, if the applied signal should take a sudden drop when it isnot on resonance, the two diode currents are very markedly decreased.The

currents on the priis instantaneously lessened even !or practically nochange in point 3! results.

. potential will vary dependin termlnes the average diode current, andthus the average damping effect, and thereby determines how low theapplied energy can drop before the self-regulating eflect ceases tooperate tain the audio potential of point 3i against the effect of aninstantaneous decrease in the signal energy applied from the amplifiertube 2. Demnduiators oi frequency-modulated waves operating inaccordance with this invention are known in the trade as ratiodetectors.

If resistor TI is too small, the impedance of the entire discriminatoris materially lessened and the gain of the amplifier tube 2 is small,and the discriminator output is thus low. However, ii. resistor 21 istoo large, the restoring characteristic for sudden decreases in appliedenergy is insufllcient. I have found that a good value for resistor I1is that which causes a decrease in the direct current potential of pointto to from onethird to one-tenth of the potential which exists when theresistor is removed, with the same I. F. applied to the grid of tube 2.This value upon the normal "(2 and parimpedance of the primary inparticular and of extent. In practice, primary tuned impedance of100,000 ohms at the resonant frequency a value of 25,000 ohms forresistor 21 will operate in the manner described. The resistors 25 andshould be of the order of to l megohm in this circuit.

Another advantage of the circuit shown in Fig. i is that the diode loadresistors are eflectively in parallel for audio frequency voltages, and,therefore. the difllculties due to differences in the time constants ofthe two load resistors and their associated capacities do not exist. Forexample, condensers l8 and it, which are the secondary tuningcondensers, form a time constant circuit with resistors 25 and 26 inparallel. The time constant can, if desired, be equal to microsecondsand thereby provide de-emphasis of the detected FM signal. In presentstandard FM broadcasting there occurs a pre-emphasis of the highermodulation frequencies during modulating or the FM carrier wave at thetransmitter. Accordingly, it will be seen that the secondary tuningcondensers and the load resistors may cooperate to perform an additionalfunction, to wit: tie-emphasis of the higher modulation frequencies.

I have heretofore stated that in one embodiment of my present inventionthe direct current voltage across the series-connected load resistors isestablished at a predetermined fixed value. This bias may be provided byeither a battery, or by the bleeder oi a direct current voltage sourcewhich may be a part of the direct current supply network of thereceiver. In Fig. 3 I have shown a modification of the invention whereinthe condenser and its shunt resistor 21 are replaced by a source ofsubstantially fixed voltage. It will be understood that the essentialdifference between the arrangement of Fig. 3 and that shown in Fig. 1consists in the aforesaid replacement. The arrows through coils iii andI6 indicate that the inductance value of these coils may be varied. Inthis modification the bleeder resistor 32 is connected between thecathode 22 of the diode l3 and the grounded anode 23 of diode It. Theresistors 32 and 33 are included in the usual voltage supply dividerwhich is connected between the +8 terminal and ground. This voltagedivider 32, I3 is constructed so that there is developed across resistorI! the desired direct cur- I 1 rent voltage which is to function as thesubstantially i'lxed voltage between point 30 and ground.

impulses occurring between the predetermined FM station settings of thetuning device will be oi insufllcient magnitude to cause detection.

Upon the applied signals overcoming the delay bias on diodes II and II,the D.-C. potentials across resistors 25 and 28 will continue to remainat a total the voltage source 32 imposes a substantially fixed value onthe total voltage between fixed and self-bias.

I do not wish to be limited to specific values of resistors, condensers,and other elements since those skilled in the art will have muchlatitude in the choice of such values. However, in a circuit accordingto Fig. 1 which was successfully operated by me the following elementshad the values indicated:

Resistor 2I=500,000 ohms Resistor 2B=500,000 ohms Resistor 2l=25,000ohms Condenser =4 microiarads Likewise, the circuit shown in Fig. 3 wassuccessiuily operated with elements having the following values:

Resistor 2l=500,000 ohms Resistor 2l=500fl00 ohms Resistor I2=1,000 ohmsResistor 33=50.000 ohms Resistor 3I=between 100.000 ohms and 250,000

ohms

Condenser =40 mid.

1 The elements included in resonant circuits I and" a can have the 12 I,and make the primary ll inductance to allow it to be nance with the oisuiilcientiy iigh brought into resonormal shunt tube and circuitcapacities. thus producing a very high tuned impedance.

In Fig. 4 I have shown how to employ the present PM detector circuit toprovide automatic volume control (AVG) bias for the receiver. To

Fig. i. That is is connected to point the cathode of the diode isconnected to The anode I4 is connected to point 3i, while of the diodeis grounded. The condenser 14 connects point III to ground, as in Fig.

pointed out in the discussion of Fig. 1 that the direct current voltagesacross resistors 25 and II are in additivepolarity relation. Fig. 4shows how the resulting direct current blocking condenser ll. ends ofcoil voltages E oi Fig. i.

In Figs. 1 and 4 resistor I! and condenser It constitute a longtime-constant circuit connected with the diodes II and it throughconducting leads 0! negligible impedance to flow or electric current.The same is true of the circuit II, 34 of Fig. 3 and the circuit 12, I3.15 of Fig. 5. The arrangements of Figs. 1, 4 and 5 also operate withoutanode potential other than that provided by the impressed signal waves.Moreover, as has been heretofore fully explained, the arrangements ofFigs. 1, 4 and 5 provide a direct current voltage which changesautomatically in accordance with the average strength of the receivedsignals to the proper value required to receive frequency modulatedwaves with simultaneous rejection of undesired amplitude variations.

In Fig. 6 I have portrayed in a purely explanatory manner the differencebetween the actions of the circuits of Fig. l and Fig. 3 respectively.The detected output voltage of an FM signal wave is plotted against theFM signal input amplitude. The dashed line 80 is the curve of outputversus input for the circuit connection shown in Fig. 1, while solidline if shows the respective relations for the circuit of Fig. 3.

Consider, first, a signal input strength represented by a on theabscissa. The detected output voltage for the circuit of Fig. 1 would berepresented by the point a on the ordinate, but the detected output fromthe circuit of Fig. 3 would be zero, since in the latter case amplitudeat is insufficient to override the fixed bias on diodes l3 and II inFig. 3. Consider, next, an amplitude represented by b on the abscissa.In this case the detected output in the circuit of Fig. 1 would berepresented by b on the ordinate, and the detected output of the circuitof Fig. 3 would be h". It can, therefore, be seen that the detectedoutput of the circuit of Fig. 1 will be directly proportional to the FMsignal input. However. because of the aforementioned action whereincondenser 24 inhibits rapid variations in the bias of diodes I3 and Iland thereby obliterates rapid amplitude variations of the I. F.potentials applied to those diodes, rapid variation in the am litude ofthe signal would cause the output potential of point 3| Fig. 1 to followeither of the dotted curves 83 or 83' depending upon the averagestrength of the applied signal. As previously explained, the size ofresistor 21 in relation to the tuned impedance of circuit 5 and otherelements of the circuit will determine the fraction of the averageamplitude to which the signal must momentarily drop before it hits theknee of the curve as indicated at either 84 or B5.

The curve 8|, as previously mentioned, is indicative of the action ofthe circuit of Fig. 3. In this case no output is realized unless thesignal amplitude is greater than that indicated by a, and increasingoutput with input will be experienced until the signal is of sumcientamplitude to pass the knee of this curve at 86. From here on furtherincrease in amplitude, either on a slow or fast basis, causes no changein detected output. This action occurs, because of the fact that point30 is rigidly fixed at a given potential (in this case positive) withrespect to ground, and this potential is divided equally as a bias ondiodes I3 and M in a direction which inhibits conduction unless theapplied I. F. vector sum potentials exceed these fixed direct currentpotentials as will be the case at point a. For stronger signals, such asthose represented by b, the bias on point 30 is still rigidlymaintained, and the aforementioned damping action is very greatlyincreased due to excessive diode currents. The net result is that thevector potentials applied to the diodes are no larger than would be thecase had potentials Just sufilcient to cause those vectors to overridethe knee ll of the curve 8| been applied at the grid I of amplifier 2.

While I have indicated and described several systems for carrying myinvention into effect. it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention.

What I claim is:

1. In a detector for angle modulated waves, a pair of rectifiersarranged in a closed loop in like current-flowing sense, a respectiveload resistor operatively associated with each rectifier, means forcombining the rectified direct current potentials across said resistorsin series-aiding polarity, and means for maintaining the sum of the tworectified potentials across said resistors against modulation frequencyvariations.

2. In a balanced frequency modulation detector of the type employing aPair of two-electrode rectifiers, means for combining the rectifiedcurrent potentials of the rectifiers in series-aiding polarity, andmeans for maintaining the sum of the rectified potentials againstmodulation frequency variations.

3. In combination with a source of angle modulated carrier waves, meansfor deriving from the waves a pair of signal voltages whose relativemagnitudes are dependent upon the angular modulations of the waves, apair of rectifiers including respective load resistors, means forapplying each of said pair of voltages to a respective rectifier, meansincluding a circuit having resistance and capacity in parallel with eachother and having negligible impedance for modulation frequency currentsfor connecting said rectifiers in parallel for modulation frequencycurrents, means for connecting said rectifiers in series-aiding polarityfor direct current potentials, and means for deriving modulationfrequency voltage variations from across at least one of the rectifiers.

4. In combination, a source of frequency modulation signals, adiscriminator therefor, a pair of diode rectifiers, means connecting theanode of one rectifier and cathode of the second to predetermined pointsof said discriminator, a condenser of low impedance to audio frequencycurrents connecting the cathode of said one rectifier to the anode ofthe second rectifier, and means for deriving modulation signal voltagesfrom across at least one of the rectifiers.

5. In combination, a source of frequency modulation signals. adiscriminator therefor, a pair of diode rectifiers, means connecting theanode of one rectifier and cathode of the second to predetermined pointsof said discriminator, a condenser of low impedance to audio frequencycurrents connecting the cathode of said one rectifier to the anode ofthe second rectifier, means for deriving modulation signal voltage, aload resistor shunting each respective diode, said deriving means beingconnected to one of the load resistors.

6. In a detector for frequency modulated waves, a pair of dioderectifiers arranged in a series-aiding closed loop, a respective loadresistor shunted across each rectifier, means for combining therectified direct current potentials across said resistors inseries-aiding polarity, and a condenser for maintaining the sum of thetwo rectified potentials across said resistors l-fllnst modulationfrequency variations.

7. In a frequency modulation detector of the type employing a pair oftwo-electrode rectifiers, means for combining the rectified currentpotentials of the rectifiers in series-aiding polarity, and means forconnecting the rectifier circuits in parallel for modulation frequencycurrents and for maintaining the sum of the rectified potentials againstmodulation frequency variations.

8. In combination with a source of frequency modulated carrier waves,discriminator means for deriving from the waves a pair of signalvoltages whose relative amplitudes are dependent upon the frequencydeviations of the waves, a pair of diode rectifiers including respectiveshunt load resistors, means for applying each of said pair of voltagesto a respective diode rectifier, a condenser for connecting said dioderectifiers in parallel for modulation frequency currents, means forconnecting said rectifiers in seriesaiding polarity for direct currentpotentials, and adjustable means for deriving from one of the loadresistors modulation frequency voltage variations.

9. In combination, a source of frequency modulation signals, adiscriminator therefor, a pair of two-electrode rectifiers, meansconnecting the anode of one rectifier and cathode of the second topredetermined spaced points of said discriminator, a condenser of lowimpedance to audio frequency currents connecting the cathode of said onerectifier to the anode of the second rectifier. means for deriving fromone rectifier modulation signal voltage, and means for deriving from thesecond rectifier a direct current voltage variable with signalamplitude.

10. In combination, a source of frequency modulation signals, adiscriminator therefor, a pair of diode rectifiers. means connecting theanode of one rectifier and cathode of the second to predetermined pointsof said discriminator. a condenser of low impedance to audio frequencycurrents connecting the cathode of said one rectifier to the anode ofthe second rectifier, means for deriving from one rectifier modulationsignal voltage, a separate load resistor shunting each respective diode,said deriving means including an adjustable tap connected to one of theload resistors.

11. In combination, a source of frequency modulation signals, adiscriminator therefor, a pair of diode rectifiers, means connecting theanode of one rectifier and cathode of the second to predetermined pointsof said discriminator, a condenser of low impedance to audio frequencycurrents connecting the cathode of said one rectifier to the anode ofthe second rectifier, a resistor in shunt across said condenser. andmeans for deriving audio voltage from said rectlfiers.

12. In combination, a source of frequency modulation signals, adiscriminator therefor, a pair of diodes, means connecting the anode ofone diode and cathode of the second diode to predetermined points ofsaid discriminator, a condenser of low impedance to audio frequencycurrents connecting the cathode of said one diode to the anode of thesecond diode, a load resistor shunting each respective diode, and aresistor in shunt with said condenser.

13. In a system as defined in claim 12, capacitive means operativelyassociated with said load resistors to produce a time constant of suchvalue as to decrease the higher modulation frequency component voltages.

14. In a detector for angle modulated waves of the type embodying a pairof two-electrode rectifiers and wave input network supplying a pair ofsignal potentials whose relative ampli- 3 tudes are a function of theangle modulation of said waves: the method which includes applying saidpotentials to respective rectifiers, adding the respective rectifieddirect current potentials in series-aiding relation, and maintaining thetotal of the added potentials against modulation frequency variations.

15. In a detector for angle modulated waves of the type embodying a pairof rectifiers and wave input network supplying a pair of signalpotentials whose relative amplitudes are a function of the anglemodulation of said waves; the method which includes applying saidpotentials to respective rectifiers, adding the respective rectifieddirect current potentials in series-aiding relation, and maintaining thetotal of the added potentials against modulation frequency variationsand variably damping said input network as a function of theinstantaneous amplitude of the signal applied to said network.

18. In combination with an amplifier for angle modulated carrier wavescomprising at least one vacuum tube, means for deriving from the waves apair of signal voltages whose relative magnitudes are dependent upon theangular modulations of the waves. a pair of rectifiers, means forapplying each of said pair of voltages to a respective rectifier, meansfor connecting said rectifiers in series-aiding polarity for directcurrent potentials. means for substantially maintaining the sum of therectified potentials against modulation frequency variations, said meansallowing the sum potential to vary at a rate slower than usefulmodulation frequency variations, and means for utilizing such sumvoltage for automatic control of the gain of said vacuum tube amplifier.

IV. In combination with a source of angle modulated carrier waves, meansfor deriving from the waves a pair of signal voltages whose rela- 5 tivemagnitudes are dependent upon the angular modulations of the waves, apair of rectifier-s including load resistance. means for applying'eachof said pair of voltages to a respective rectifier,

means for connecting said rectrfiers in series- M aiding polarity fordirect current potentials, means for utilizing in parallel themodulation frequency components of the rectified outputs of saidrectifiers, and capacitive means associated with the load resistance toproduce a time conas stant to decrease the higher modulation frequencycomponent voltages.

18. In combination with a source of frequency modulated carrier waves,an input circuit including means for deriving from the waves a pair of00 signal voltages whose relative magnitudes are dependent upon thefrequency modulations of said waves, a pair of rectifiers each having atleast an anode and a cathode, a circuit connecting the anode of onerectifier to the cathode 08 of the other, another circuit connecting theoathode of the first-mentioned rectifier to the anode of thesecond-mentioned rectifier, said last-mentioned circuits in series withsaid rectifiers constituting a closed loop for flow of uni-directional10 currents and one of said last-mentioned circuits including at least apart of the input circuit, means for applying each of said pair ofvoltages to a respective rectifier, a resistor in one of said circuits,means for establishing across said resis- 75 tor a direct currentpotential substantially fixed 17 against amplitude variations atmodulation frequencies variations, and means connected between saidcircuits for deriving modulation-irequency voltage variations therefrom.

19. In combination with a source oi frequency modulated carrier waves,means for deriving from the waves a pair of signal voltages whoserelative magnitudes are dependent upon the frequency modulations of saidwaves, a two-electrode rectifiers each having an anode and a cathode, acircuit connecting the anode of one rectifier to the cathode of theother, a second circuit connecting the cathode of the first-mentionedrectifier to the anode of the second-mentioned rectifier, said circuitsin series with said rectifiers constituting a closed loop for fiow ofuni-directional currents, means for applying each of said pair ofvoltages to a respective rectifier, a resistor in one of said circuits,and means for establishing across said resistor a direct currentpotential substantially fixed against amplitude variations at modulationfrequencies variations, the desired modulation frequency variationsbeing developed on one of said circuits in relation to the other.

20. A detector comprising a pair of two-elements rectifiers each havingan anode and a cathode, a circuit connecting the anode of one rectifierto the cathode of the other, a second circuit connecting the cathode ofthe first-mentioned rectifier to the anode oi the second-mentionedrectifier, said circuits in series with said rectifiers constituting aclosed loop for flow of in one oi said circuits, means ior establishingacross said resistor a direct current potential substantially fixedagainst modulation frequency variations, means for derivingmodulation-irequency voltage variations from one of said circults inrelation to the other, and means for automatically controlling the gainof said amplifier by the potential obtained from said resistor, whichvaries with the average strength of the voltages applied to therectifiers.

23. In a demodulator of angle-modulated waves, two rectifiers eachhaving at least an anode and a cathode arranged in series for flow ofdirect current, means including a common input circuit also connected inseries with the rectifiers for flow of direct current for impressingmodulated carrier frequency waves on said rectillers, means forestablishing on one side of said rectifiers a direct current potentialstabilized against voltage variations of modulating frequency, andconnections across said rectifiers for deriving modulation frequencyvoltages from said demodulator.

uni-directional currents, means for applying modulated carrier waves tothe respective rectifiers, means in one of said circuits forestablishing a direct current potential substantially fixed againstamplitude variations at modulation frequencies variations, and means forderiving modulation-frequency voltage variations from one of saidcircuits in relation to the other.

21. In combination with a source of ansle modulated carrier waves, meansfor deriving from the waves a pair of signal voltages whose relativemagnitudes are dependent upon the modulations of said waves, atwo-electrode rectifiers each having an anode and a cathode, a circuitconnecting the anode of one rectifier to the cathode oi the other, asecond circuit connecting the cathode of the first-mentioned rectifierto the anode of the second-mentioned rectifier said circuits in serieswith said rectifiers constituting a closed loop for fiow ofuni-directional currents, means for applying each of said pair ofvoltages to a respective rectifier, a resistor in one of said circuits,means for establishing across said resistor a direct current potentialsubstantially fixed against amplitude variations at modulationfrequencies variations, and means for deriving modulationfrequencyvoltage variations from one of said circuits in relation to the other.

22. In combination with an amplifier for frequency modulated carrierwaves comprising at least one vacuum tube. means for deriving from thewaves a pair of signal voltages whose relative magnitudes are dependentupon the frequency modulations of said waves, a pair of rectifiers eachhaving at least an anode and a cathode, a circuit connecting the anodeof one rectifier to the cathode of the other, a second circuitconnecting the cathode of the first-ment oned rectifier to the anode oithe second-mentioned rectifier said circuits in series with saidrectifiers constituting a closed loop for flow of unisdirectionalcurrents. means for applying each of said pair of voltages to arespective rectifier, a resistor 24. A detector comprising a pair ofrectifiers each having at least an anode and a cathode, a circuitconnecting the anode of one rectifier to the cathode of the other, asecond circuit connecting the cathode of the first-mentioned rectifierto the anode of the second-mentioned rectifier, said circuits in serieswith said rectifiers constituting a closed loop for flow ofuni-directional currents, means for applying modulated carrier waves tothe respective rectifiers, means in one of said circuits forestablishing between terminals of the rect fiers and by anode-cathodecurrents produced by the modulated carrier waves a direct currentpotential substantially fixed against modulation frequency variations,and means for deriving modulation-frequency voltage variations from oneof said circuits in relation to the other.

25. In a demodulator of angle-modulated wav two rectifiers each havingat least an anode and a cathode arranged in series for flow of directcurrent, means for impressing modulated carrier frequency waves on saidrectifiers, means in the connections on one side of said rectifiers forestablishing a direct current voltage varying in value in accordancewith the average strength of the impressed modulated carrier waves andstabilized against voltage variations of modulating frequency, andconnections across said rectifiers for deriving modulation frequencyvoltages from said demodulator.

26. In a demodulator of angle-modulated waves, two rectifiers eachhaving at least an anode and a cathode arranged in series for flow ofdirect current, means for impressing modulated carrier frequency waveson said rectifiers, a resistance and capacity in parallel with eachother connected between the rectifiers to establish by anode-cathodecurrents produced bv the modulated carrier waves a direct currentvoltage stabilized against voltage variations of modulating frequency,low impedance connections between said resistor and said rectifiers andconnections across said rectifiers for deriving modulation frequencyvoltages from said demodulator.

2'1. In combination with a source of frequency modulated carrier waves,a pair of rectifiers each having at least an anode and a cathode, acircuit connecting the anode of one rectifier to the cathode of theother, a second circuit connecting the cathode of the first-mentionedrectifier to the anode of the second-mentioned rectifier, one of saidcircuits including means for impressing on said rectifiers a pair ofsignal voltages whose relative magnitudes are dependent upon thefrequency modulations of said waves and said circuits in series withsaid rectifiers constituting a closed loop for flow of uni-directionalcurrents, a resistor in one of said circuits, a large condenserconnected across said resistor for establishing a direct currentpotential substantially fixed against modulation frequency variations,connections of substantially zero impedance between said resistor andadjacent terminals of the rectifiers, and means for derivingmodulation-frequency voltage variations from one of said circuits inrelation to the other.

28. In combination with a source of anglemodulated carrier waves, meansfor deriving from the waves a pair of signal voltages whose relativemagnitudes are dependent upon the modulations of the waves, said meansincluding a primary circuit, a secondary circuit tuned to the carrierfrequency and two couplings between the primary and secondary circuits,one of which transfers the angle-modulated waves to the secondarycircuit without phase change and the other of which transfersangle-modulated waves to the secondary circuit with a voltage phasechange dependent on the modulations of the waves, a pair of rectifierseach having at least an anode and a cathode, a direct current conductingcircuit connecting the anode of one rectifier to the cathode of theother, a second direct current conducting circuit connecting the cathodeof the first-mentioned rectifier to the anode of the second-mentionedrectifier, one of said direct current conducting circuits including saidsecondary circuit and connections for applying each of said pair ofvoltages to a respective rectiller and said direct current conductingcircuits in series with said rectifiers constituting a closed loop foriiow oi uni-directional currents, resistance in one of said directcurrent conducting circuits, a large capacity in parallel with saidresistance for establishing across it a direct current potentialsubstantially fixed against audio frequency variations, and connectionsfor deriving audio frequency voltages from across said rectifiers.

29. In a demodulator of angle-modulated waves, two rectifiers eachhaving at least an anode and a cathode arranged in series for flow ofdirect current, means for impressing modulated carrier frequency waveson said rectifiers, a resistance of the order of 25.000 ohms and acapacity of the order of 4 microfarads connected in parallel with eachother between the rectiflers to establish a direct current voltagedepending in value on the average strength of the impressed carrierwaves and stabilized against voltage variations of modulation frequency,low impedance connections between said resistance and said rectifiers,and connections across said rectiflers for deriving modulation frequencyvoltages from said demodulator.

30. A method of detecting angle modulated waves which comprisesdeveloping from the waves a pair of voltages of wave frequency whoserelative amplitudes are a function of the instantaneous frequency,deriving in response to said pair of voltages a pair of respectiveunidirectional voltages, maintaining the sum oi the unidirectionalvoltages substantially constant against modulation frequency variat onsbut in proportion to the average strength of the received waves,substantially maintaining the ratio of said unidirectional voltages indirect proportion to 'said relative amplitudes, and comparing theunidirectlonal voltages to provide an indication of the ratio of saidrelative amplitudes.

31. In a detector for angle modulated carrier waves, a discriminator forderiving from the waves a pair of signal voltages whose relativeamplitudes are dependent upon the modulations of said waves. a pair oftwo-electrode rectifiers, a circuit connecting the anode of onerectifier to the cathode of the other, a second circuit connecting thecathode of the first-mentioned rectifier to the anode of thesecond-mentioned rectiller, said circuits in series with said rectiiiersconstituting a closed loop for flow of undirectional currents,connections in one of said circuits for applying one of said pair ofvoltages to one of the rectifier-s and the other of said pair ofvoltages to the other rectifier, a resistance in the other of saidcircuits. a capacity for maintaining the potential produced byrectification of received angle modulated waves substantially fixedagainst modulation frequency variations, and connections for derivingmodulation-frequency voltage variations existing between the two saidcircuits.

32. In combination with an amplifier for angle modulated waves, adiscriminator for deriving from the waves a pair of signal voltageswhose relative amplitudes are dependent upon the angular modulations ofthe waves, a pair of rectiflers, connections for applying one oi saidvoltages to one of said rectifiers and the other of said voltages to theother rectifier, circuits connecting said rectifiers in series-aidingpolarity for direct current, a potential source constant againstamplitude variations at modulation frequencies. said potential sourceallowing the sum of the rectified potentials to vary at a rate slowerthan useful modulation frequency variations, and connections forutilizing such sum potential for automatic control of the gain of saidamplifier.

33. In combination with an amplifier for angle modulated waves, adiscriminator for deriving from the waves a. pair of signal voltageswhose relative amplitudes are dependent upon the angular modulations ofthe waves, a pair of twoelectrode rectifiers. connections for applyingone of said voltages to one of said rectifiers and the other of saidvoltages to the other rectifier, circuits connecting said rectifiers inseries-aiding polarity for direct current, a potential source constantagainst amplitude variations at modulation frequencies, said potentialsource allowing the sum of the rectified potentials to vary at a rateslower than useful modulation frequency variations, connections forderiving modulation frequency voltages from 'the rectitlers, andconnections for utilizing such sum potential for automatic control ofthe gain of said amplifier.

34. In combination, a discriminator for angle modulated waves, saiddiscriminator including a secondary circuit for deriving from the wavesa pair of voltages whose relative amplitudes are dependent upon theangular modulations of the waves, a pair of two-electrode rectiflers,two circuits connecting said rectifiers in series for direct currents,one of said circuits including said discriminator secondary circuit andthe other of said circuits including a potential source constant againstamplitude variations at modulation frequencies but responsive to theaverage amplitude of the waves applied to the input circuit oi thediscriminator, and an output circuit 21 for deriving modulation signalvoltage: from said Number two rectifier-connecting circuits. 2,190,319STUART w. ;2,282,101 8,286,378 REFERENCES CITED 5 52 29 4 The followingreferences are of record in the 3302334 file of this patent: 12,413, 13

UNITED STATES PATENTS Number Name Date be 2,173,231 Koch Sept. 19, 1939552, 07

Home Data Koch Peb. 13. 1040 Tunick May 5. 1942 Roberts June 16. 1942Crosby Sept. 16, 1942 Bliss Nov. 24. 1942 Duke Jan. I. 1947 FOREIGNPATENTS Country Dnte Great Britain Apr. 27, 1948

